1. Field of the Invention
The present invention relates to a carrier sensing method and RFID transceiver device using the same.
2. Description of the Related Art
RFID (radio frequency identification) systems are employed in various applications. In such systems, a carrier signal is transmitted from an interrogator to a transponder, the carrier signal that is reflected (back-scattered) from the transponder is received, and the modulation signal component that is contained in the back-scattered carrier signal is processed as information data from the transponder.
The interrogator is an RFID transceiver device called a reader/writer (RW). The transponder may be of various types, one of which is an IC tag. Furthermore, the RFID system shares the frequency band which it uses for communication with the tag with other RFID receivers or other communication devices so, in order to avoid collision, it is necessary for the RFID system to confirm prior to transmission that the frequency that the system plans to use itself is not being employed by another system. This is called carrier sensing.
FIG. 1 is a view given in further explanation of carrier sensing. Prior to transmission, the RFID transceiver device 1A that is preparing to transmit ascertains whether or not any other RFID transceiver devices 1B are already in communication with an IC tag 1C by detecting the presence of the carrier signal that is exchanged between the RFID transceiver devices 1B and the IC tag 1C.
FIG. 2 is an example of the block diagram of an RFID transceiver device. A signal processing circuit 10 that is connected through the external interface (I/F) with a data processing device, not shown, controls a local oscillation circuit 11 to generate a local oscillation signal corresponding to the channel that is used.
Specifically, as shown in FIG. 3, the RFID system uses a plurality of channels (10 channels in the example of FIG. 3) in for example a 2 MHz frequency band. The control and signal processing circuit 10 performs control such that a local oscillation signal of frequency corresponding to one of the channels of this plurality of channels is output from the local oscillation circuit 11.
In this RFID transceiver device block diagram shown in FIG. 2, when carrier sensing is performed, transmission output from a transmission circuit 12 is suspended, in order to confirm that the frequency (channel) that the RFID transceiver device itself plans to use is not being used by another RFID transceiver device.
When a reception circuit 14 receives a carrier signal of the frequency that is planned to be used corresponding to the local oscillation frequency that is output from the local oscillation circuit 11, the reception circuit 14 outputs the received demodulated signal to the control and signal processing circuit 10. When the control and signal processing circuit 10 receives the received demodulated signal from the reception circuit 14, it assumes that it is impossible to use a channel wherein a carrier signal is already in existence and successively shifts the frequency of the local oscillation signal that is output from the local oscillation circuit 11 until it can find a free channel.
When it thus finds a free channel, as shown in FIG. 4, the RFID transceiver device performs communication in the communication period P2 following the period P1 of carrier sensing (CS), using the carrier frequency of the free channel that has been found, for communication with the tag. The transmission circuit 12 modulates the carrier frequency signal that is output from the local oscillation signal generating circuit 11 with the command signal before emitting it from the transceiving antenna 16 through the duplexer 13.
The corresponding tag modulates the received carrier frequency signal with information data and transmits this as a response signal to the RFID transceiver device. The RFID transceiver device acquires the information data by demodulating the response signal that is transmitted back thereto.
While the RFID transceiver device executes communication with the IC tag in this way, it is undesirable that communication using a specified carrier frequency should be performed exclusively by a specified RFID transceiver device. Control is therefore effected so as to free the channel after lapse of a fixed time, by restricting the transmission period (period P2).
The block diagram of the reception circuit 14 is assumed to be a DC directly coupled reception system (FIG. 5A) or AC coupled reception system (FIG. 5B) as shown in FIG. 5.
Let us assume that, in the carrier sensing period (P1 in FIG. 4) another system is performing communication using the frequency that is planned to be used. The frequency of the carrier signal of the other RFID transceiver device that is in the course of communication (for example 1B in FIG. 1), being input to the demodulator 140 included in the reception circuit 14, is (fL0+Δf). The frequency offset Δf is the frequency difference caused by the fact that the RFID transceiver device that is currently preparing to transmit (for example 1A of FIG. 1) and the RFID transceiver device 1B that is currently communicating have reference oscillation sources that are independent of each other.
In the block diagram of the reception circuit 14 shown in FIG. 5A, the output of the local oscillation circuit 11 (frequency fL0) and the reception signal of frequency (fL0+Δf) are mixed in the demodulating circuit 140. The frequency offset component Δf then appears at the output of the demodulating circuit 140. This frequency component Δf is therefore amplified by amplifier 141 and input through a low pass filter 142 to the control and signal processing circuit 10 after being converted to a corresponding digital signal by means of an analog/digital converter 143.
In this way, it is possible for the control and signal processing circuit 10 to identify whether the channel in question is in use by another RFID transceiver device even if Δf is a frequency component close to “0”.
Since, in the case where the IC tag is a passive tag, the operating power (power source energy) is obtained from the electromagnetic wave transmitted by the RFID transceiver device, the RFID transceiver device needs to have large transmission power. In contrast, since the response transmission from the IC tag is performed by back scattering, its power is very weak in comparison with the power of the electromagnetic wave transmitted by the RFID transceiver device.
Thus, the RFID transceiver device whose communication partner is a passive IC tag needs to have high output power in order to supply power source energy to the IC tag and, at the same time, must be provided with a high sensitivity reception capability, since the back-scattered signal from the passive IC tag is very weak.
Also, providing the RFID transceiver device with separate antennas for transmission and reception is undesirable from the point of view of cost and size. A transceiving antenna 16 is therefore employed. A duplexer 13 that isolates the route of the transmission and reception signal and that is connected with the common antenna 14 is therefore provided. By means of the duplexer 13, carrier signals from the transmission circuit 12 are fed to the antenna 14 and back-scattered signals from the IC tag received by the antenna 14 are fed to the reception circuit 14.
Inventions related to such an RFID system are disclosed in for example U.S. Pat. No. 6,639,509 and in U.S. Pat. No. 6,122,329.
U.S. Pat. No. 6,639,509 discloses a configuration in which carrier demodulation is performed with the object of reducing high frequency componets in a reception circuit of an RFID transceiver device.
Also, the invention disclosed in U.S. Pat. No. 6,122,329 makes it possible to reproduce a back-scattered data signal using an RFID transceiver device (interrogator) in a condition accompanied by abrupt movement of the tag (transponder).
As described above, in an RFID transceiver device, the energy of the carrier signal that is output from the transmission circuit 12 is large, since high output is demanded in order to supply power source energy to the IC tag: this results in a leakage component 15 to the reception circuit 14 passing through the duplexer 13. When this leakage component is input to the demodulating circuit 140, a high level DC component is output from the demodulating circuit 140, causing saturation in the downstream amplifier and other circuits.
In a typical reception circuit 14, as shown in FIG. 5B, the DC component is therefore removed by for example AC coupling achieved by providing a capacitor 144 on the output side of the demodulating circuit 140. There is therefore the problem that, when carrier sensing is performed, if the frequency offset Δf from the other RFID transceiver device is close to “0”, carrier sensing cannot be accurately performed due to the effect of DC component removal.
Furthermore, neither the above U.S. Pat. No. 6,639,509 nor U.S. Pat. No. 6,122,329 discuss the problem of carrier sensing.